The Ideal Transformer Equation (Edexcel GCSE Physics: Combined Science)

• An ideal transformer would be 100% efficient
• Although transformers can increase the voltage of a power source, due to the law of conservation of energy, they cannot increase the power output
• If a transformer is 100% efficient:

Input power = Output power

• The equation to calculate electrical power is:

P = VI

• Where:
  • P = power in Watts (W)
  • V = potential difference in volts (V)
  • I = current in amps (A)

• Therefore, if a transformer is 100% efficient then:

V_p × I_p = V_s × I_s

• Where:
  • V_p = potential difference across primary coil in volts (V)
  • I_p = current through primary coil in Amps (A)
  • V_s = potential difference across secondary coil in volts (V)
  • I_s = current through secondary coil in Amps (A)

• The equation above could also be written as:

P_s = V_p × I_p

• Where:
  • P_s = output power (power produced in secondary coil) in Watts (W)

Step 1: List the known quantities

• • Voltage in primary coil, V_p = 115 V
  • Voltage in secondary coil, V_s = 230 V
  • Current in secondary coil, I_s = 5 A

Step 2: Write the equation linking the known values to the current drawn from the supply, I_p

V_p × I_p = V_s × I_s

Step 3: Substitute in the known values

115 × I_p = 230 × 5

Step 4: Rearrange the equation to find I_p

Step 5: Calculate a value for I_p and include the correct unit

I_p = 10 A